Ph.D. in Environmental Physics for the University of Girona. Nowadays PostDoc researcher in the Institute for Geophysics and Meteorology in the University of Cologne (Germany). Member of High Definition of Clouds and Precipitation for Climate Prediction (HD(CP)²) Project.

T'agrada:

Accounting for wavelength effects will likely improve climate models

RICHLAND, Wash. – Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.

Additionally, the researchers found that sunlight scattered by clouds — the reason why beachgoers can get sunburned on overcast days — is an important component of cloud contributions to the earth’s energy balance. Capturing such contributions will increase the accuracy of climate models, the team from the Department of Energy’s Pacific Northwest National Laboratory reported in Geophysical Research Letters earlier this month.

“The amount of the sun’s energy that reaches the earth’s surface is the main driver of the earth’s temperature. Clouds are one of the least understood aspects of climate change. They can block the sun, but light can also bounce off one cloud into another cloud’s shadow and increase the solar energy hitting earth,” said PNNL atmospheric scientist Evgueni Kassianov.

Cloudy dissonance: These cumulus clouds above Oklahoma both shade the earth and make shadows brighter. The larger ones have a distinct cauliflower shape, providing even more opportunities for light to bounce off of them.

White clouds

Clouds both cool down and warm up the earth’s surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.

But it’s a hard number to get. Fair-weather clouds are big puffy white objects that bounce a lot of light around. They can make the sky around them look brighter when they’re there, but they float about and reform constantly. Cloud droplets and aerosol particles in the sky — tiny bits of dirt and water in the air that cause haziness — scatter light in three dimensions, even into cloud shadows.

To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky’s brightness. The difference between those numbers is the net cloud effect.

Rainbow energy

Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth’s surface, from ultraviolet to infrared. But clouds are white — that’s because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.

On the other hand, aerosols — both within clouds and in the open sky — bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.

Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color — violet, green, orange, red — and two of infrared.

In addition, this instrument, a spectral radiometer at DOE’s Atmospheric Radiation Measurement Climate Research Facility located on the southern Great Plains in Oklahoma, allowed the team to calculate what the brightness would be if the day sported a cloudless, blue sky. The spectral measurements taken by the radiometer can be converted into the amount and properties of aerosols. Then aerosol properties can be used to calculate clear blue sky brightness.

Clouds Gone Wild

Comparing measured values for cloudy sky to the calculated values for clear sky, the researchers found that, on average, puffy fair-weather clouds cool down the earth’s surface by several percent on a summer day. Although clouds cool overall, two components that the researchers looked at — from direct and scattered sunlight — had opposite effects.

The direct component accounts for the shade provided by clouds and cools the earth. The second component accounts for the sunlight scattered between and under clouds, which makes the sky brighter, warming the earth.

“The sunlight scattered by clouds can heat the surface,” said Kassianov. “We all know that we can still get sunburned on cloudy days. This explains why.”

In the Oklahoma summer, the scattered-light effect measured by the researchers could be quite large. For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being “caught on tape” by the radiometer.

“Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale climate models don’t capture this diffuse light,” said Kassianov.

Aerosols’ Day in the Sky

The team also found that the effect changed depending on the measured visible-spectrum wavelength, and whether the light was direct or scattered.

With direct light, the cooling caused by clouds was weakest on the violet end of the spectrum and strongest at infrared. With scattered light, warming caused by clouds was also weakest at violet and the strongest at infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.

Because large droplets in clouds scatter sunlight almost uniformly across the spectrum, the clouds themselves can’t be the reason why different wavelengths contribute differently to the net cloud effect. Compared to cloud droplets, aerosols are more than 100 times smaller and scatter wavelengths differently. These results suggest that aerosols — which not only cause haziness but contribute to cloud formation as well — are responsible for the wavelength differences, something researchers need to be aware of as they study clouds in the sky.

“If you want to study how aerosols and clouds interact,” said Kassianov, “you need to look in the region of the spectrum where aerosol effects are significant. If you want to fish, you go where the fish are biting.”

Pacific Northwest National Laboratory is a Department of Energy Office of Science national laboratory where interdisciplinary teams advance science and technology and deliver solutions to America’s most intractable problems in energy, the environment and national security. PNNL employs 4,900 staff, has an annual budget of nearly $1.1 billion, and has been managed by Ohio-based Battelle since the lab’s inception in 1965. Follow PNNL on Facebook, LinkedIn and Twitter.

T'agrada:

I want to tell you some things about this Research Laboratory that I’m staying in a brief stay for four months.

It is located in Richland, on the sunny eastern side of Washington state. PNNL is one of the U.S. Department of Energy’s (DOE’s) ten national laboratories, managed by DOE’s Office of Science. PNNL also performs research for other DOE offices as well as government agencies, universities, and industry to deliver breakthrough science and technology to meet today’s key national needs.

The Laboratory

provides the facilities, unique scientific equipment, and world-renowned scientists/engineers to strengthen U.S. scientific foundations for fundamental research and innovation

prevents and counters acts of terrorism through applied research in information analysis, cyber security, and the non-proliferation of weapons of mass destruction

increases U.S. energy capacity and reduces dependence on imported oil through research of hydrogen and biomass-based fuels

reduces the effects of energy generation and use on the environment.

PNNL currently has approximately 4,900 staff members and a business volume of more than $1.1 billion. The William R. Wiley Environmental Molecular Sciences Laboratory, a DOE Office of Science national scientific user facility, is located on PNNL’s Richland campus. PNNL operates a marine research facility in Sequim, and has satellite offices in Seattle and Tacoma, Washington; Portland, Oregon; and Washington, D.C.

Battelle has operated PNNL for DOE and its predecessors since 1965. A unique feature of Battelle’s contract with DOE allows the staff to work for private industry. To learn more about PNNL, see the video, Advancing Transformational Science and Technology on PNNL’s YouTube channel and read the transcript.

Concretly, I’m in the Fundamental & Computational Sciences Directorate, at the Atmospheric Science & Global Change Division and in the Climate Physics Group.

This challenge is addressed by a combination of field measurements and modeling as part of the DOE Atmospheric Radiation Measurement program.

Climate and Cloud Physics

Global Climate modeling can be enhanced by the use of satellite and surface field measurements. Pacific Northwest National Laboratory researcher Chuck Long (my mentor) has developed the Total Sky Imager to remotely provide vital field measurements in real time. TSI uses a digital camera to collect hemispheric ”fish eye” images of the sky, which are then processed to infer what amount of the sky contains clouds at the monitored site.

Develop new process parameterization schemes for community climate models.

Develop and evaluate new high-resolution global climate models.

Develop instrumentation and conduct field observations for improving understanding of cloud processes and effects.

Much of the uncertainty in projections of global climate change is due to the complexity of clouds, aerosols, and cloud-aerosol interactions, and the difficulty of representing them in climate models. This challenge is addressed by a combination of field measurements and modeling as part of the DOE Atmospheric Radiation Measurement program. Improved instrumentation and methods for determining cloud and aerosol properties from surface and satellite measurements are developed. These retrievals are used to evaluate the clouds and aerosols simulated by global climate models. The evaluation guides further development of the cloud and aerosol parameterizations for the global climate models.